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Topic: What does a transistor do in a complex LCR circuit? (Read 3365 times)

so ive just realized upon revisiting the site there is no Science of electricity and circuitry hmmm.. a queation i cant seem to find an answer to when dealing with an LCR circuit with several Z factors what would happen if you through a transistor into a z factor? how would that affect the other branch currents etc.

Throw a transistor in is rather a vague idea as transistors are three terminal devices and can fit into your circuit in various ways.Transistors are very nonlinear devices on the input side with the current flow starting after about 0.7v for silicon devices then increasing at an exponential rate, the current available on the output circuit normally varies at a more or less fixed ratio to the input current about 100 times in low voltage devices to about 2 times in high voltage devices.As you can imagine mathmetical analysis of circuits including transistors can be quite complex as their capacitance also varies with voltage.

Tony, I don't think anyone understood your question. By Z-factors I guess you mean complex impedances. As Syhprum says, "throwing in" a transistor is too imprecise to have any meaning. Circuits with transistors in them can still be analysed courtesy of Ohm and Kirchoff however, provided you include suitable models of the transistors. Transistors behave non-linearly so it is usual to do such modelling for small signals at a fixed DC condition.

sorry im still in school for advanced avionic electronics and still dont know everything, when calculating values on a complex 3 branch LCR circuit what effect would putting a transistor into a a branch with an inductor and a capacitor?

As Graham and Syhprum pointed out, it's not possible to answer that without knowing a lot about how the transistor was configured, and what else it was connected to. If only two of the leads on the transistor are hooked up, it would pretty much look like a diode of some sort or other, and as that has a highly non-linear current/voltage relationship, it would be difficult to predict what would happen without a good model of the transistor.

I'm very out of touch with this stuff, but I guess you might be able to find a circuit modeling program on the web that will let you test some ideas. Anybody aware of any?

For a free schematic entry tool and simulator, Simetrix is quite good. It limits the number of components and some features unless you pay money, but it is reasonably powerful, and would show what any circuit configuration does. This does not help the understanding of the maths involved in small signal analysis of course.

As a general purpose mathematical analysis tool MatLab is very good but expensive. An excellent free alternative is SciLab - but don't expect it to be easy to use. And whilst on the subject of free software, as a general mathematical equation manipulator then wxMaxima is very good, but again non-trivial to use.

Basic Matlab is a few thousand quid (without the optional packages). As I said, these programs are good but are not trivial to use. They require a large investment in time to learn to use. SciLab is the nearest to MatLab and has the benefit of being free!

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